It is current practice to use a formed metal linecutter adjacent the propshaft of small marine engines such as outboards, inboards and inboard-outboards, used for fishing. The purpose of the linecutter is to protect the propshaft seal(s) from nylon fishing line which may wrap around the propshaft. The inner diameter of the linecutter must be ground for two reasons: (1) to produce a sharp outward-facing edge to cut fishing line, and (2) to achieve a minimal shaft clearance with an exceptionally tight tolerance (.+-.0.0005-0.0010 inches).
The linecutter shell is usually cup-shaped and the outer propshaft seal is pressed into it to form a seal-linecutter assembly. Therefore, the inside diameter-outside diameter concentricity of the metal linecutter must be held to 0.005 inch TIR (total indicator reading) or less. The major disadvantages of such known linecutters are cost, grooving of the shaft due to shaft runout, and generation of heat near the seal.